linux-kernel - Re: [RFC v2 1/1] clk: Add notifier support in clk_prepare/clk

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Date:	Fri, 15 Mar 2013 18:58:23 -0700
From:	Bill Huang <bilhuang@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC v2 1/1] clk: Add notifier support in
 clk_prepare/clk_unprepare

On Sat, 2013-03-16 at 03:51 +0800, Stephen Warren wrote:
> On 03/15/2013 11:45 AM, Russell King - ARM Linux wrote:
> > On Thu, Mar 14, 2013 at 02:31:04AM -0700, Bill Huang wrote:
> >> Add the below two notifier events so drivers which are interested in
> >> knowing the clock status can act accordingly. This is extremely useful
> >> in some of the DVFS (Dynamic Voltage Frequency Scaling) design.
> >>
> >> CLK_PREPARED
> >> CLK_UNPREPARED
> >>
> >> Signed-off-by: Bill Huang <bilhuang@...dia.com>
> >> ---
> >>  drivers/clk/clk.c   |    3 +++
> >>  include/linux/clk.h |    2 ++
> >>  2 files changed, 5 insertions(+)
> >>
> >> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> >> index ed87b24..3292cec 100644
> >> --- a/drivers/clk/clk.c
> >> +++ b/drivers/clk/clk.c
> >> @@ -550,6 +550,7 @@ void clk_unprepare(struct clk *clk)
> >>  {
> >>  	mutex_lock(&prepare_lock);
> >>  	__clk_unprepare(clk);
> >> +	__clk_notify(clk, CLK_UNPREPARED, clk->rate, clk->rate);
> >>  	mutex_unlock(&prepare_lock);
> >>  }
> >>  EXPORT_SYMBOL_GPL(clk_unprepare);
> >> @@ -598,6 +599,8 @@ int clk_prepare(struct clk *clk)
> >>  
> >>  	mutex_lock(&prepare_lock);
> >>  	ret = __clk_prepare(clk);
> >> +	if (!ret)
> >> +		__clk_notify(clk, CLK_PREPARED, clk->rate, clk->rate);
> > 
> > So, on prepare, we notify after we've prepared the clock.  On unprepare,
> > we notify after the clock has been shut down.  Are you sure that's the
> > correct ordering?  Would it not be better to view it in a stack-like
> > fashion, iow:
> 
> > get
> > 	prepare
> > 		notify-prepare
> > 			enable
> > 			disable
> > 		notify-unprepare
> > 	unprepare
> > put
> 
> Yes, these should be stacked/nested better for consistency.
> 
> But for DVFS, the voltage needs to be raised before the prepare body is
> run, so that if clk_prepare actually enables the clock, the voltage is
> already at the safe level required by that clock. Similarly, for
> unprepare, you can only lower the voltage after having turned off the
> clock, which is guaranteed after the unprepare body. So, I think you
> want to move the notifier for prepare in the code above (and rename it
> to pre/before_prepare?), rather than the notifier for unprepare.

Oh yes I should raised notify before prepare body is run.
> 
> In order to cover more cases, you might have both
> {pre,post}_{un,}prepare notifiers, although I'm not sure when you'd use
> the other two options.

Right, maybe {pre,post}_{un,}prepare will be useful.
> 
> >> diff --git a/include/linux/clk.h b/include/linux/clk.h
> 
> >> +#define CLK_PREPARED			BIT(3)
> >> +#define CLK_UNPREPARED			BIT(4)
> 


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